Wood is one of the most important constructional materials available. However, wood has a great affinity for water, and it shrinks and swells with changes in humidity. This dimensional instability of wood is a major problem associated with its use as a structural material.
Green timber as felled contains substantial amounts of water, most of which is driven off when the timber is dried for use. The water which is most readily lost on drying is held within the cell voids or lumens. On drying to a moisture content of about 20-25% by weight, the "fiber saturation point" is reached. At that point, the remaining water is held within the cell wall substance. Further drying, for example to a moisture content of about 12%, is accompanied by a partial collapse of the cell walls and an associated overall shrinkage in the wood dimensions of about 7-10% of the fully swollen volume. This effect is essentially reversible, so that dried wood exposed to high relative humidities or to liquid water re-expands.
The dimensional instability of wood can lead to splitting, warping and grain raising. In addition, the dimensional changes are unevenly distributed through the wood substance, being greater for the relatively dense summer or late wood than for the spring or early wood. This differential leads to high localized stresses in the wood, especially at the junction of successive seasons' growth, which can contribute significantly to paint failures.
A variety of approaches have been attempted to dimensionally stabilize wood. Exemplary of such techniques are surface coating or painting of wood to attempt to prevent the entry of water; chemical modification of wood; and cross-linking of wood. Another approach to dimensional stabilization of wood aims at replacing the water in wood with non-volatile chemicals. This is termed "bulking".
Various compositions have been reported for use as wood bulking compositions. Included among such compositions are polyethylene glycol, salts, sugars, glycols, waxes, phenol-formaldehyde and urea-formaldehyde systems. Chemicals such as sugars, salts and polyethylene glycol have been shown to provide good bulking, i.e., dimensional stability. However, they are readily leached out by moisture causing the bulking effect to be lost if the wood is exposed to high humidities. In addition, the presence of large amounts of polyethylene glycol in wood appears to impart a wet feeling to the wood surface and, further, retards the drying or curing of many types of paint films.
None of the known compositions are completely satisfactory, and the search for alternative and improved bulking compositions has continued. The present invention provides new curable compositions having an excellent combination of properties making them suitable for use as timber bulking compositions. Moreover, the compositions of this invention remain stable for prolonged periods at ambient temperatures. In another aspect, the present invention provides a method of bulking timber utilizing the aforementioned compositions.